Weak precipitation δ2H response to large Holocene hydroclimate changes in eastern North America

In eastern North America, annual precipitation increased by >40% over the Holocene, largely in response to melting of the Laurentide Ice Sheet. The change substantially raised lake levels and transformed conifer-dominated ecosystems into mesic deciduous forests. The δ2H of terrestrially derived leaf-wax n-alkanes can facilitate diagnoses of the climate dynamics involved by reconstructing the δ2H of mean annual precipitation (δ2HMAP). However, competing influences on δ2HMAP in the mid-latitudes, such as changes in moisture sources and in the seasonal distribution of precipitation, can generate confounding effects. To test the sensitivity of the δ2H of mean annual precipitation to potential changes associated with the final Holocene phases of deglaciation in eastern North America, we used 14 fossil-pollen records to reconstruct monthly precipitation changes and to model δ2HMAP during the Holocene. The pollen-inferred precipitation increased by 100-200 mm during both cold and warm seasons, but modelled δ2HMAP changed by only ~10‰ because isotopically-heavy summer precipitation increased by nearly as much as the cold-season isotopically-light winter precipitation. Three new C29 alkane (δ2HC29) records spanning the Holocene from Vermont, Pennsylvania, and Massachusetts closely follow the modeled δ2HMAP trends and confirm only a small decline in δ2HMAP. Because the shifts in precipitation seasonality accurately predict the n-alkane records, changes in moisture sources or pathways appear to play only a minor role in the regional δ2HMAP history despite the effects of deglaciation on atmospheric circulation. Soil evaporation also did not significantly alter δ2HC29 from the values predicted from the pollen-derived reconstructions. The results affirm that δ2HC29 faithfully detected anticipated isotopic changes in δ2HMAP, but that important paleoclimate events may not always yield strong changes in δ2HMAP.